The invention relates to novel heat-curable thixotropic mixtures containing carbamate and/or allophanate groups. The present invention further relates to processes for preparing the novel heat-curable thixotropic mixtures containing carbamate and/or allophanate groups. The present invention relates additionally to the use of the novel heat-curable thixotropic mixtures containing carbamate and/or allophanate groups as coating materials, adhesives and sealing compounds. The present invention relates in particular to the use of the novel heat-curable thixotropic coating materials containing carbamate and/or allophanate groups as clearcoat material, especially for producing clearcoats as part of multicoat color and/or effect coating systems.
Thixotropic materials and coating materials comprising them have been known for a long time.
The use of thixotropic agents in coating materials is intended, inter alia, to allow comparatively thick paint films to be applied without disruptive running. Especially in the case of nonaqueous coating materials comprising a thixotropic agent based on urea derivatives, and at least at high solids contents, the resulting film surfaces are unsatisfactory in terms of their visual appearance (especially leveling and gloss) and, moreover, lead to coatings lacking “condensation resistance” (exhibiting blushing owing to water infiltration). Thixotropic agents based on silicas give coating materials which are likewise unsatisfactory in terms of leveling.
A key feature of thixotropic agents is that the viscosity of a coating material prepared using them depends on the flow history and/or that the thixotropic agents are pseudoplastic, i.e., the viscosity of the coating material decreases as the applied shear stress goes up. Starting from a baseline viscosity, the viscosity decreases under shear stress and returns only gradually to the initial level after the shear stress has been removed. A thixotropic gel, for example, liquefies as a result of input of mechanical energy (stirring or the like) and solidifies again only gradually after the end of the input of energy. Pseudoplastic or thixotropic properties are advantageous for paint processing. In particular, the tendency of a coating material to run on application at high wet-film thickness may be controlled and reduced. On the other hand, thixotropic agents must not adversely affect the optical and chemical properties of a finished coating produced using them. As a general rule, thixotropic agents are particulate and are present in dispersion in a coating material, whether aqueous or nonaqueous. In the case of the urea derivatives, these thixotropic agents are acicular crystals, in part with a helical twist, for which, preferably, a particle size distribution between 0.1 μm and 6 μm (95-99% of the particles, based on the volume) is established and 80% of the crystals (based on the number) are smaller than 2 μm.
Coating materials comprising constituents containing allophanate groups are known from the German patent application DE 198 39 453 A1. These coating materials are capable of wide application and are outstandingly suitable for the wet-on-wet technique for producing multicoat color and/or effect coating systems. The coatings produced from the coating materials are highly compatible with other coatings and are scratch-resistant and weathering-stable. Providing these known coating materials with thixotropic agents is not described in the German application.
The international patent application WO 00/31194 discloses thixotropic agents comprising urea derivatives and polymers containing lateral or terminal carbamate groups. The urea derivatives may be prepared by reacting monoamines with polyisocyanates in the presence of these polymers. The coating materials provided with the thixotropic agents may also comprise surface-active or interface-active substances (surfactants; cf. Römpp Lexikon Lacke und Druckfarben, Georg Thieme Verlag, Stuttgart, New York, page 271, +“Surface-active substances”). According to example 1, page 23 line 22, of the international patent application, a silicone-glycol copolymer (Worlee® 315) is used. The thixotropic agents improve the thixotropic behavior of the coating materials provided with them and effectively suppress the tendency to run. The coatings produced from them exhibit high gloss and a high level of hardness.
The German patent application DE 199 46 048 A1 discloses coating materials whose binder comprises at least one resin containing lateral and/or terminal carbamate groups and preparable by reacting a resin containing at least one hydroxyl group with phosgene to give a resin containing at least one chloroformate group, which resin is then reacted with ammonia, primary amines and/or secondary amines. Apart from the customary and known crosslinking agents for heat curing, the coating materials may further comprise numerous different additives. For instance, besides numerous other additives, mention is made of sag control agents (thixotropic agents), such as ureas and/or modified ureas or silicas, as described for example in the aforementioned literature references EP 0 192 304 A1, DE 23 59 923 A1, DE 18 05 693 A1, WO 94/22968, DE 27 51 761 C2; WO 97/12945 and farbe+lack, 11/1992, pages 829 ff. Precise details as to the amounts in which these additives are to be used are lacking. It is merely stated that the additives are added in customary and known, effective amounts, which depending on the additive are from 0.001 to 500 parts by weight per 100 parts by weight of resin and crosslinking agents. In the examples, however, these additives are not used.
The known coating materials have numerous advantages which, however, can be attributed to the specific binders or resins containing carbamate groups.
The international patent application WO 00/37520 discloses urea derivatives preparable by reacting at least one amine with at least one in the presence of at least one amino resin, as thixotropic agents. The thixotropic agents are intended to be capable of being employed universally. Their use in heat-curable thixotropic mixtures containing carbamate and/or allophanate groups is not described in the international patent application. Nor does the patent application reveal whether—and if so to what extent—the thixotropic agents influence the scratch resistance of coatings, adhesive films and seals produced from the thixotropic coating materials, adhesives and sealing compounds in question.
The heat-curable thixotropic coating materials, adhesives and sealing compounds containing carbamate groups which are known to date, especially the clearcoat materials, therefore have a number of technical advantages, which ought to be retained in the course of their further development. Despite the high technological level attained, the coatings, adhesive films and seals produced from the heat-curable thixotropic coating materials, adhesives and sealing compounds containing carbamate groups which are known to date still leave something to be desired in terms of their scratch resistance and reflow characteristics.
It is an object of the present invention to provide novel heat-curable thixotropic mixtures containing carbamate and/or allophanate groups that are suitable for use as coating materials, adhesives and sealing compounds, these compositions attaining the high technological level of the prior art, if not indeed exceeding it. Furthermore, the novel heat-curable thixotropic coating materials, adhesives and sealing compounds containing carbamate and/or allophanate groups should give coatings, adhesive films and seals which attain the high technological level of the prior art, if not indeed exceed it, and at the same time possess high scratch resistance and very good reflow characteristics.